1. Field of the Invention
The present invention is related to a regulation circuit, especially to a regulation circuit associated with a synchronous rectifier providing cable compensation for the power converter.
2. Description of the Related Art
FIG. 1 shows a prior art of a power converter. A PWM controller (PWM) 30 generates a switching signal SPWM to switch a transformer 10 having a primary winding NP and a secondary winding NS via a power transistor 20 in accordance with a feedback signal VFB for regulating the output of the power converter. The primary winding NP of the transformer 10 is coupled to receive an input voltage VIN. The feedback signal VFB is generated by an opto-coupler 60 in response to the output voltage VO of the power converter. The opto-coupler 60 is controlled by an error amplifier 50. The error amplifier 50 generates a signal VF coupled to control the opto-coupler 60. The error amplifier 50 includes a reference signal VR supplied with a positive input terminal of the error amplifier 50 for regulating the output voltage VO. The output voltage VO is coupled to an negative input terminal of the error amplifier 50 via a voltage divider developed by resistors 51 and 52. A capacitor 53 is coupled between the negative input terminal of the error amplifier 50 and an output terminal of the error amplifier 50.
The secondary winding NS is coupled to an output terminal of the power converter to generate the output voltage VO. A rectifier 40 is coupled to one terminal of the secondary winding NS. An output capacitor 45 is coupled to the other terminal of the secondary winding NS and the output terminal of the power converter to generate the output voltage VO. A resister 62 is coupled form the capacitor 45 and the rectifier 40 to the opto-coupler 60.
Generally, the output cable of the power converter has a voltage drop proportional to its output current. Sensing the output current to offset the voltage drop is an approach for the output cable compensation. However, it will generate a significant power loss while sensing the output current by using a shunt resistor. The present invention provides a method and apparatus to compensate the output voltage without the need of sensing the output current of the power converter by the shunt resistor.